Multiple user diversity gain in a multiple-user environment is obtained by monitoring a change in an independent fading channel for each user and by performing resource allocation when a channel for each user substantially becomes the best state through scheduling. As the change of the channel is increased, the diversity gain is increased. Accordingly, the diversity gain is limited under a channel environment where the change of the channel is small or slow.
Opportunistic beamforming (hereinafter referred to as “OBF”) uses a plurality of transmitting antennas to induce a large and fast change in an arbitrary channel, resulting in multiple-user diversity gain. Based on the possibility that a user for whom a channel is in the best state always exists is high if there are many users, OBF performs wireless resource allocation to the user for whom the channel is in the best state for a corresponding timeslot, thereby improving system capacity. However, since OBF generates only one beam every timeslot, the performance is not good, if the number of users is small.
Codebook-based opportunistic beamforming (hereinafter referred to as “COBF”) that has appeared for making up for the drawback uses a random unitary matrix having a pattern defined in existing codebook-based beamforming (hereinafter referred to as “CBF”) to obtain multiple-user diversity gain and thus has system capacity that is larger than the existing OBF or CBF. A random unitary matrix having a pattern defined in COBF induces a change in a channel without increasing transmission power to achieve an opportunistic effect, and many codewords in a codebook give diversity in beamforming vector selection to individual users, resulting in excellent performance even if the number of users is small.
However, since OBF and COBF use multiple transmitting antennas to transmit one stream every timeslot, they cannot make full use of the spatial degree of freedom of multiple antennas.
Meanwhile, opportunistic multiple beamforming (hereinafter referred to as “OMBF”) which is a space division multiple access (hereinafter referred to as “SDMA”) scheme of simultaneously transmitting multiple streams to a plurality of users using the same resource through multiple beams by multiple antennas, and PU2RC (per user unitary rate control) which is an SDMA scheme using a codebook including a plurality of unitary matrixes, are multiple user MIMO (multiple-input multiple-output) schemes that increase the availability of the spatial degree of freedom, thereby improving the system capacity.
However, since OMBF should find an appropriate user with a set of multiple beams every timeslot, the performance may be deteriorated if the number of users is small, and PU2RC may not obtain sufficient multiple-user diversity gain in a channel environment in which a change in a channel is small or slow.
Further, according to a codebook-based pre-coding scheme, since the amount of channel information increases as the size of a codebook increases, the probability that each user achieves the best channel gain increases and thus the performance of the pre-coding is improved. However, if the size of the codebook is largecodeword system overhead for the feedback increases when every user terminal feeds the codeword index selected by itself back. In particular, since the overhead linearly increases as the number of users increases, serious problems may occur when a common channel is used to perform the feedback.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.